Ribosome inactivating proteins (RIPs) have been used as instruments of biological warfare and terrorism. Ricin is a heterodimeric plant toxin that consists of A and B-chains and the prototype of type II RIPs. Its B chain is a lectin that enables cell binding. After endocytosis, the A-chain translocates through the membrane to reach the cytosol where its N-glycosidase activity removes a specific adenine from the highly conserved, sarcin/ricin (S/R) loop in the large rRNA. Ricin has been classified as Category B priority for biodefense. Our group has been studying pokeweed antiviral protein (PAP), a type I RIP that consists of only the A chain, which has the same enzymatic activity as ricin. Because ricin and PAP cause apoptosis, they have been targeted to cancer cells as immunotoxins, which are potent anti-cancer agents. However, very little known about the molecular mechanism by which these toxins cause apoptosis. Translation inhibition by ribosome depurination has been hypothesized to be responsible for their cytotoxicity. We recently showed that PAP binds to the cap structure on eukaryotic mRNA and depurinates the mRNA downstream of the cap structure in vitro. PAP regulates the stability of its own mRNA and specific cellular mRNAs in vivo by a mechanism that can be dissociated from the depurination of the SIR loop. Our preliminary results indicate that PAP mRNA and specific cellular mRNAs are selectively targeted for degradation by a common mechanism, which correlates with cell death. The overall goal of this R21 application is to determine if ricin binds to the cap structure and affects the stability of its own mRNA and specific cellular mRNAs and whether it causes cell death by altering mRNA stability. Cap-binding and mRNA depurination are novel activities that have been demonstrated for the first time with PAP. Ricin differs from PAP in its subunit structure, ribosome specificity and antiviral activity. Plant ribosomes are relatively resistant to ricin, but sensitive to PAP. Unlike PAP, which has antiviral activity, ricin is not antiviral. Because of these differences, we believe the studies proposed in this application are of a highly speculative nature. We believe they will yield sufficient information about the mechanism of action of ricin upon which we will base further investigations. These studies will aid in the design of protection strategies against toxins that are classified as high-risk candidates for bioterrorism.